Combustion engines such as diesel engines typically produce unburned fuel residues or particulates i.e., soot, which is usually composed mainly of amorphous carbon. Exhaust from vehicles containing soot becomes airborne particulate matter and increases pollution, particularly in urban areas. Soot also is carcinogenic and, therefore, very hazardous to the lungs and general health when inhaled.
Diesel Particulate Filters (DPFs) have been developed to remove soot from the exhaust gas of diesel engines. After a period of time, however, enough soot may have collected on a DPF to cause an increase in pressure drop across the DPF, which results in compromised operation of the engine. Therefore, the DPF must be cleaned of accumulated soot. Some DPFs are designed for single use and are disposable, while other filters are designed to burn off the soot through carbon soot combustion or oxidation, which is known as filter regeneration. Filter regeneration may occur actively through a fuel burner that heats the soot to combustion levels or passively through the use of a catalyst.
Several types of filters have been used with various vehicles. For example, Flow-Through Oxidation Catalysts are used in filters to remove diesel particulate matter, CO and hydrocarbons, including the ones that form the soluble organic fraction of the total particulate mass. The oxidation catalyst used with Flow-Through Oxidation Catalysts is typically a platinum-rhodium-platinum catalyst deposited on a flow through monolith, where soot particles are not trapped. The oxidation catalysts convert CO and hydrocarbons at ˜200° C. but achieve less than 5% oxidation of the particulate matter. Platinum, however, is an excellent SO2 oxidation catalyst. At temperatures above approximately 300-350° C., the catalyst oxidizes SO2 to SO3, which quickly combines with water to form sulfuric acid and contributes significantly to the total particulate mass. To limit emissions above 300° C., a more specific catalyst should be used to minimize SO2 oxidation. A tailored catalyst with comparable activity may be made by alloying less active oxidation catalysts, rhodium and palladium. In addition, base metals also may be used to tailor the activity of platinum such as, for example, where only soluble organic portions of the total particulate matter need to be lowered to meet any particular emission standards.
In addition, NOx-Aided CRTs are a type of filter typically used with trucks and buses. The NOx-Aided CRT includes a wall-flow monolith with an upstream flow-through diesel oxidation catalyst, called a preoxidizer, such as a platinum catalyst, and a cordierite wall-flow monolith downstream. The preoxidizer converts 90% of the CO and hydrocarbons to CO2 and 20-50% of the NO to NO2; the particles are trapped on a cordierite wall-flow monolith and subsequently oxidized by the NO2. NOx-Aided CRTs effectively oxidize all of the carbon components in diesel exhaust that include small particles and unregulated compounds. In addition, NOx-Aided CRTs reduce NOx concentration by approximately 3-8%. Furthermore NOx-Aided CRTs have a reasonable temperature window of approximately 200-450° C. (200° C. is needed for CO and hydrocarbon oxidation, and 450° C. relates to the chemical equilibrium between NO and NO2, which is not favorable above 450° C.). Also, NOx-Aided CRTs have higher stability because of the continuous regeneration, which avoids extreme temperatures and enhances stability. However, these NOx-Aided CRT systems also have some limitations including requiring low-sulfur fuel, which makes wide-scale introduction unfeasible.
For light-duty vehicles such as, for example, passenger cars, an Integrated Catalytic Trap filter may be used that includes a silicon carbide wall-flow monolith, engine-controlled heating through fuel-injection-timing controls, cerium fuel additives and a preoxidizer (e.g., platinum catalyst). These systems include two catalyst technologies and make several catalyst mechanisms available including, for example, cerium-aided periodically induced self-supporting regeneration; cerium-catalyzed spontaneous local regeneration reactions at low temperatures; cerium-catalyzed continuous soot oxidation at high temperatures; cerium-catalyzed reduction of black smoke after some initial cerium deposition in the combustion chamber and exhaust system (cerium fuel additive reduces the raw particulate emissions by approximately 20%); platinum-catalyzed oxidation of volatile hydrocarbons and CO; platinum-catalyzed production of NO2 at favorable temperatures; and platinum- and cerium-catalyzed synergetic oxidation of soot. However, this system is oftentimes complex and expensive and the trap should be cleaned periodically to remove cerium deposits.
Furthermore, alkali metals such as potassium improve the activity of catalysts for carbon soot combustion. Unfortunately, because of the high temperatures needed for carbon soot combustion and because traditional K2CO3, KOH, and KO2, potassium catalysts typically have low thermal stability, if the potassium is mobile and not tightly bound in the compound (i.e. if it is “free” potassium) potassium may be lost via evaporation or sublimation, etc. This results in a subsequent reduction in desired catalytic activity and, therefore, limits the usefulness of some alkali metal containing compounds as catalysts if there is no way to replenish the catalyst over time. This is true in a DPF environment and in other industries such as, for example, in coal gasification reactions wherein potassium-based catalysts are quickly consumed as a result of the combustion process.
U.S. Pat. No. 6,631,612 describes a device and method of filter regeneration used to avoid a reduction in desired catalytic activity. The device and method described in U.S. Pat. No. 6,631,612 adds seawater containing alkali metals to the filter to replenish the alkali metals used a catalyst in the filter regeneration. This device and method require a readily available supply of seawater, which is impractical for land-based vehicles. Furthermore, the device and method described in U.S. Pat. No. 6,631,612 require constant, repetitive steps, i.e., adding seawater, which can be onerous. This is an example of “active” catalyst replenishment, as opposed to “passive” replenishment as detailed herein.